Step 1: Disassembly

Yours will vary depending on the manufacturer of the modem. As you can see I first had to remove the rubber feet covering the screw heads. Unscrew then pry the case apart.

Then pop the circuit board out of the top housing.

To make the fan fit I removed the top cover(gray piece in the third picture) it was just held in with snaptabs.

Step 2: Soldering

To do this I used a pick tool to disassemble the connector on the cpu fan to expose the clips you see in the picture. I found the tabs using a multimeter. This is my first time I have ever soldered so its pretty sloppy I have only ever done plumbing pipe before.

Step 3: Reassemble

Luckly the fan I chose fit perfectly into the case without need for any kind anchors.
I plugged it in and it has been running great ever since.

Future Improvements
Add a POT cause the fan running at full blast can be kinda loud at night(its right next to my bed)
Search out a specific fan the can be mounted inside the case